Task history, implementation of https://github.com/spotify/luigi/issues/51

All the tables are added as per the RFC, though currently
only Task, TaskEvent, and TaskParameters is being used right now.

Task history is added by the scheduler, history viewer
available through /history/
Merge branch 'task_history' of git://github.com/JoeEnnever/luigi into JoeEnnever-task_history

Conflicts:
	test/customized_run_test.py
Merge pull request #132 from erikbern/task-history

Task history